The steering and manipulation of ion beams for low-energy heavy ion accelerators

Abstract

Both electrostatic and magnetic fields are used in low-energy accelerators. Electrostatic fields are essential in the acceleration stages and they are commonly used for ion beam scanning and focussing. Magnetic field are only infrequently used as lenses, but they are essential for mass analysis and are sometimes employed for beam steering. The electrostatic mirror is a versatile and compact lens which has hitherto received little attention for the controlled manipulation of heavy ions. In addition to energy analysis it can be used to steer, focus and scan beams and its flexibility and usefulness can be further increased by shaping the electrostatic field in the mirror space. The use of a computer programme to model the focussing behaviour of a variety of lens shapes is described and it is shown that the focal properties of the mirror can be controlled to produce a parallel, convergent or divergent output beam. The use of mirrors for two-dimensional beam focussing is also outlined. To permit the use of the mirror system with heavy ions an apertured front plate, without field-defining gauzes, was utilised. In consequence an additional electrode was incorporated in the lens structure to prevent penetration of the positive electric field along the beam axes outside the mirror space. This factor and the compact design of the mirror, contributed to the minimisation of space-charge defocussing effects which normally militate against the use of such electrostatic lenses with high intensity ion beams. The results of experiments confirming the computer predictions are briefly described and, in conclusion, some possible applications of electrostatic mirrors in electromagnetic isotope separators and low energy accelerators are outlined.